I use a modified version of the related topics mod; the accuracy of its results depends on the quality of your search table and the topic titles. Adding lots of stopwords and twiddling vague titles helps. It has the positive effect of reviving long dormant discussions and finding FAQs (though new members will sometimes fail to note they're replying to a post over a year old!).

The Similar Topics MOD matches the topic title against other topic titles; the Related Topics MOD uses the search table to match the keywords from the first post against other topic titles; it will be more costly in terms of SQL calls. Related Topics also tends to go "farther afield" than Similar Topics; depending on the quality of your topic titles and search index, this can be a good or bad characteristic.